The present invention relates to an air-fuel ratio control system for use in an internal combustion engine for an automobile or the like.
Heretofore, varieties of air-fuel ratio control systems for controlling the air-fuel ratio of the air-fuel mixture supplied to the combustion chamber of an automotive internal combustion engine are in use for fuel cost reduction or as an anti-exhaust measure. As disclosed by Japanese Patent Application (OPI) No. 59-196946, for instance, there is known a so-called feedback carburetor system equipped with a carburetor and a fuel control solenoid and used to converge the oxygen content of the exhaust gas up to the stoichiometric air-fuel ratio under feedback control by means of an oxygen sensor (O.sub.2 sensor) during normal operation after the internal combustion engine is warmed up and to set the air-fuel ratio of an air-fuel mixture at a predetermined set value under open loop control at low temperatures, i.e., at the time of warming-up, starting, heavy-loading or slowing-down.
In such a feedback carburetor system, the air-fuel ratio at the time of acceleration is corrected by the quantity of fuel flow from the acceleration pump incorporated in the carburetor to improve operating performance.
However, since the acceleration pump is mechanically operated, the quantity of fuel flow is normally determined by the opening of the throttle and, when the system is operated under diversified operating conditions as in the case of an automobile, an optimum quantity of supply fuel can hardly be set in accordance with different accelerating conditions. Admittedly, rather excessive fuel will have to be normally supplied in view of operating performance and it is therefore difficult to meet highly developed demands of drivers in terms of operating performance and fuel cost.